The present disclosure relates generally to drivers and driving methods for light emitting diodes (LEDs).
Superior in power conversion efficiency, compact product size, and life span, LEDs are broadly popular in the industries of house lighting and panel backlight. For example, a great number of LCD backlight panels are currently using LED modules for backlight, rather than CCFL modules that were commonly adopted several years ago.
FIG. 1 demonstrates a LED driver 10 capable of being used in a backlight module. The LED driver 10 drives a LED string 12 consisting of LEDs connected in series. In the LED driver 10, a booster 18 converts power source VIN at a major power line to power source VOUT at an output power line OUT. A LED string 12 and a current switch 22 are connected in series between the output power line OUT and a ground line GND. A power controller 14 periodically turns ON and OFF a power switch 16 to control the power conversion of the booster 18. While powering the output power line OUT, the major purpose of the power controller 14 is to stabilize a feedback voltage VFB at the feedback node FB, equivalently stabilizing the current flowing through the LED string 12 and the brightness of the LED string 12.
A dimming signal SDIM is fed to the enable node EN of the power controller 14. A level shifter 20 shifts the dimming signal SDIM with a logic level of 5V to become a switch signal SMOSDIM with a logic level of 12V. When the dimming signal SDIM is 5V in voltage level, or “1” in logic, the current switch 22 is ON, performing a short circuit, and the power controller 14 periodically switches the power switch 16 to regulate the feedback voltage VFB. Accordingly, the LED string 12 illuminates stably.
When the dimming signal SDIM is 0V in voltage level, or “0” in logic, the current switch is OFF, performing an open circuit, and the power controller 14 constantly turns OFF the power switch 16. As there is no power converted to power the LED string 12, it darkens.
The design of the LED driver 10 shall take several abnormal events, such as LED open, LED short, output over voltage, or flickering, to name a few, into consideration. For example, if power source VOUT at an output power line OUT is over high, it might impose electric shock to careless operators or fire accident to environment, such that output over voltage should be prevented.